Not applicable.
The present invention is directed to an electrokinetic pump wherein the porous dielectric medium is comprised of a patterned microstructure fabricated by lithographic patterning and etching of a substrate. The microstructure can be comprised of features arranged so as to create an array of microchannels whose dimensions are on the order of the pore spacing in a conventional porous dielectric medium.
Electrokinetic pumps (EKP) are devices for converting electrical potential to hydraulic power. They comprise generally at least one tube or channel, that can be a capillary channel or microchannel, forming a fluid passageway containing an electrolyte and having a porous dielectric medium disposed therein between one or more spaced electrodes. The porous dielectric medium can include small particles, high surface area structures fabricated within the microchannel, or microporous materials. An electric potential is applied between the electrodes that are in contact with the electrolyte, that can be aqueous or an organic liquid or mixtures thereof, to cause the electrolyte to move in the microchannel by electro-osmotic flow. The electric field applied across the EKP by the electrodes will cause the electrolyte contained in the porous dielectric medium to flow and if presented with an external flow resistance will create a pressure at the down stream end of the EKP. The flowrate of the electrolyte is proportional to the magnitude of the applied electric field (V/m applied across the EKP) and the pressure generated is proportional to the voltage across the device. The direction of flow of the electrolyte is determined by both the nature of the electrochemical interaction between the porous dielectric medium and the electrolyte and the polarity of the applied electric potential. Moreover, an EKP can be realized by integrating part or all of the described components on a chip or micro-scale device, i.e., a device wherein the components have features with dimensions less than about 0.1 mm. Thus, the EKP is a compact and efficient device that converts electric power to hydraulic power in the working fluid and has been shown to be capable of generating hydraulic pressures greater than 10000 psi. A detailed discussion of the theory and operation of the electrokinetic pumping process can be found in prior co-pending U.S. Pat. Nos. 6,013,164 and 6,019,882, both entitled ELECTROKINETIC HIGH PRESSURE HYDRAULIC SYSTEM, assigned to the same assignee, and incorporated herein by reference in their entirety.
One example of a porous dielectric medium used in an EKP is a packed bed of dielectric particles that have a diameter of between 100 nm and 5 xcexcm and form a bed having a pore size of between about 2-200 nm.
One problem associated with using particulate materials as the porous dielectric medium is packing capillary tubs or microchannels for use on microchips. As the channel diameter decreases it becomes more difficult to pack the microchannel in a uniform and reproducible way. Irregularities in the uniformity of the porous dielectric, both along the length and across the diameter of the column, affects device performance.
For particles between 1 and 20 xcexcm in diameter slurry techniques can be used. In slurry packing the particles that form the bed are suspended as a slurry in an appropriate liquid or liquid mixture. Many liquids or liquid mixtures can be used to prepare the slurry, the principal requirement being that the liquid thoroughly wet the packing particles and provide adequate dispersion of the packing material. The slurry is then pumped into the microchannel. However, as the diameter of the column or channel decreases it becomes necessary to apply higher pressures to force the slurry into and through the column and pressures of 200 to 500 atm are not uncommon. In addition to the obvious hazard of having to work with very high pressures exerted on relatively thin walled structures, there are other disadvantages to this method of microchannel packing. When the pumping pressure is released at the conclusion of the packing operation the restraining force on the particle bed is partially lost causing an expansion of the particle bed. Then, when the microchannel is once again pressurized, heterogeneities or irregularities, can occur in the particle bed.
Instead of pressure, electro-osmotic flow can be used to carry particles into a capillary or microchannel from a reservoir of particles suspended in solution. This method of packing capillary columns suffers the disadvantages of needing very high voltages and a pre-formed porous plug for operation. A porous plug or other particle retaining means must be installed at the exit end of the microchannel prior to filling to prevent the particles from passing directly through the channel during the filling operation. Porous plugs are difficult to fabricate for microchannels, generally requiring that the material that composes the porous plug be positioned somehow at the appropriate place in the microchannel. The material is sintered to form a plug that must retain structural integrity as well as a high degree of porosity, while simultaneously fusing the plug to the wall of the capillary.
In general, none of the aforementioned methods generate packed beds with optimal uniformity and they can require relatively complicated hardware to perform.
Accordingly, the present invention provides an array of microchannels as the porous dielectric medium for electrokinetic pumps. The microchannels, that can be formed by conventional lithographic patterning and etching of a substrate, have dimensions on the order of the pore spacing in packed porous beds of dielectric particles. Embedded unitary electrodes are vapor deposited on either end of the channel structure to provide the electric field necessary for electroosmotic flow.